During the reaction of methyl chloride with silicon a mixture of chlorosilanes is produced. These chlorosilanes are normally separated by fractional distillation. Two of the largest volume chlorosilanes produced by this method are methyltrichlorosilane and dimethyldichlorosilane. In order to prepare satisfactory siloxane polymers from dimethyldichlorosilane it is usually necessary that the methyltrichlorosilane content of dimethyldichlorosilane be less than about 500 parts per million. The boiling points of these materials are sufficiently close that distillation columns of 200 or more stages are required to satisfactorily separate these materials in commercial operation. Consequently at the present time a large capital investment is required in order to install these columns and it would highly desirable to reduce this capital investment. Also, a large column generally requires more energy to operate than does a smaller column.
The copending application Ser. No. 06/419,854 of Ora L. Flaningam and Roland L. Halm, filed on Sept. 20, 1982, the same data and assigned to the same assignee as this present application, discloses a method by which the separation of close-boiling chlorosilanes can be carried out in a more efficient and less costly manner. The method involves the separation of close-boiling chlorosilanes by the procedures of extractive distillation using sulfolane as the extractive solvent where the lower-boiling chlorosilane is distilled from a mixture of the close-boiling chlorosilanes and sulfolane and thereafter separating the higher-boiling chlorosilane and the sulfolane. Sulfolane is tetrahydrothiophene-1,1-dioxide. Flaningam et al. discloses that the higher-boiling chlorosilane can be separated from the mixture of the higher-boiling chlorosilane and sulfolane by such techniques as gas-liquid chromatograph and distillation. Clearly, for a commerical chlorosilane plant, distillation would be the more viable procedure. Flaningam et al. further teaches that in large scale operations it is preferred that sulfolane be employed in amounts greater than 15% by weight with the preferred range being 50-90% by weight based on the total weight of chlorosilanes in the extractive distillation column. Based on the teachings of Flaningam et al. one skilled in the art would realize that in the commercial extractive distillation of chlorosilanes, using the solvent sulfolane, rather large amounts of sulfolane would be required and the process would be most attractive if the sulfolane could be recycled. For the process to be most efficient the recycled sulfolane should be virtually free of the higher-boiling chlorosilane. Otherwise, because in the preferred operation of the Flaningam et al. method the sulfolane is introduced near the top of the extractive distillation column, a significant amount of the higher-boiling chlorosilane in the sulfolane would go overhead with the lower-boiling chlorosilane. In such a situation the overall efficiencies of the process would be reduced. To obtain sulfolane virtually free of higher-boiling chlorosilane by simple distillation, it is required that sulfolane be heated very close to its pure component boiling point (285.degree. C. at atmospheric pressure) in the sulfolane recovery column. Unfortunately, sulfolane begins to undergo thermal decomposition at about 180.degree. C. Above about 220.degree. C. the thermal decomposition of sulfolane becomes especially rapid. Therefore, to minimize the thermal decomposition of sulfolane the temperature in the column used to separate the higher-boiling chlorosilane and sulfolane must be kept under about 180.degree. C. Under these conditions with a condensor pressure of about 1 atmosphere in the separation of dimethyldichlorosilane and methyltrichlorosilane, the recycled sulfolane can contain 3% by weight or more dimethyldichlorosilane. The introduction of such large amounts of dimethyldichlorosilane in the top of the extractive disillation column in the separation of methyltrichlorosilane and dimethyldichlorosilane can lower the overall efficiencies of the process.
One object of this present invention is to provide a method for recovering sulfolane virtually free of the higher-boiling chlorosilane from a mixture of the sulfolane and a higher-boiling chlorosilane obtained from the extractive disillation of close-boiling chlorosilanes without subjecting the sulfolane to prohibitively high temperature.
Another object of this present invention is to provide a method by which sulfolane used in the extractive distillation of close-boiling chlorosilanes can be recovered in a form and purity such the sulfolane can more efficiencly be reused or recycled in the extractive distillation process.
Still another object of this present invention is to provide an improved method for the separation of close-boiling chlorosilanes.
Other objects of the present invention will be apparent to those skilled in the art upon examination of this specification.